People who suffer from a hearing loss most often have problems detecting high frequencies in sound signals. This is a major problem since high frequencies in sound signals are known to offer advantages with respect to spatial hearing such as the ability to identify the location or origin of a detected sound (“sound localisation”). Consequently, spatial hearing is very important for people's ability to perceive sound and to interact with and navigate in their surroundings. This is especially true for more complex listening situations such as cocktail parties, in which spatial hearing can allow people to perceptually separate different sound sources from each other, thereby leading to better speech intelligibility [e.g. Bronkhorst, A. W. (2000), “The cocktail party phenomenon: A review of research on speech intelligibility in multiple-talker conditions,” Acta Acust. Acust., 86, 117-128].
From the psychoacoustic literature it is apparent that, apart from interaural temporal and level differences, sound localisation is mediated by monaural spectral cues, i.e. peaks and notches that usually occur at frequencies above 3 kHz [e.g. Middlebrooks, J. C., and Green, D. M. (1991), “Sound localization by human listeners,” Ann. Rev. Psychol., 42, 135-159; Wightman, F. L., and Kistler, D. J. (1997), “Factors affecting the relative salience of sound localization cues,” In: R. H. Gilkey and T. A. Anderson (eds.), Binaural and Spatial Hearing in Real and Virtual Environments, Mahwah, N.J.: Lawrence Erlbaum Associates, 1-23]. Since hearing-impaired subjects are usually compromised in their ability to detect frequencies higher than 3 kHz, they suffer from reduced spatial hearing abilities.
In principle, the term “frequency transposition” can imply a number of different approaches to altering the spectrum of a signal. For instance, “frequency compression” refers to compressing a (wider) source frequency region into a narrower target frequency region, e.g. by discarding every n-th frequency analysis band and “pushing” the remaining bands together in the frequency domain. In the context of this invention, this will be termed the frequency-compression approach. “Frequency lowering” refers to shifting a high-frequency source region into a lower-frequency target region without discarding any spectral information contained in the shifted high-frequency band. Rather, the higher frequencies that are transposed either replace the lower frequencies completely or they are mixed with them. In the context of this invention, this will be termed the frequency-lowering approach. In principle, both types of approaches can be performed on all or only some frequencies of a given input spectrum. In the context of this invention, both approaches are intended to transpose higher frequencies downwards, either by compression or lowering. Generally speaking, however, there may be one or more high-frequency source bands that are transposed downwards into one or more low-frequency target bands, and there may also be another, even lower frequency band (the “baseband”) remaining unaffected by the transposition.
Frequency transposition of particular frequency regions in hearings aids is a known technique for improving the benefits of hearing-aid users. For example, patent application WO 2005/015952 describes a system that aims at improving the spatial hearing abilities of hearing-impaired subjects. The proposed system discards every n-th frequency analysis band and pushes the remaining ones together, thus applying frequency compression. As a result, spatially salient high-frequency cues are assumed to be reproduced at lower frequencies.
In general, hearing aids may be of the behind-the-ear (BTE), mostly-in-the-ear (MIC), in-the-ear (ITE), completely-in-the-canal (CIC), or receiver-in-the-ear (RITE) type.
Patent application EP 1,742,509 relates to eliminating acoustical feedback and noise by synthesizing an audio input signal of a hearing device. Even though this method utilises frequency transposition, the purpose of frequency transposition in this prior art method is to eliminate acoustical feedback and noise in hearing aids and not to improve spatial hearing abilities.
Even though the above mentioned prior art methods provide improved hearing abilities for many subjects, even more hearing-impaired subjects could be helped, and it therefore remains a problem to obtain a further improvement of the effect of frequency transposition in a hearing aid and thus improved spatial hearing of hearing-impaired subjects.